In prior art is known a system for ECG monitoring as represented by the diagram in FIG. 1. In FIG. 1, the same patient P is diagrammatically represented by two pictures of the torso, where the lower picture shows a standard four-point placement of measuring electrodes R, F, L and N (so-called limb electrodes). For the sake of clarity, the upper picture separately shows a standard placement of ECG measuring electrodes, i.e. precordial electrodes V1, V2, V3, V4, V5, V6 on the patient's thorax. Together the limb electrodes and precordial electrodes form a so-called 12-lead connection system. A so-called 5-lead connection system, also used in ECG measurement, consists of limb electrodes together with one of the pre-cordial electrodes, e.g. R, F, L, N and V1.
Each measuring electrode is provided with a signal lead wire 1-10 which are further provided with first connector elements 11. The ECG monitoring system further comprises a collecting connector 13 provided with second connector elements 14 for receiving the first connector elements 11 of the signal lead wire. The collecting connector may reside either in the collecting cable 12 with an adapter 24 at its end or directly at the amplifier unit 16. The system further comprises an ECG apparatus 30.
The circuitry for measuring 5 or 12-lead ECG is presented in FIG. 2. The collecting connector 13 comprises twenty connector elements 14 in all. Placed in the upper row on the left are circular connector elements 141 for the upper part of connector elements 11 providing connections for the signal lead wires 1-5 coming from the limb electrodes R, F, L, N, and one precordial electrode V1. Connected to the circular connector elements 14 in the right-hand part of the upper row are the upper part of connector elements 11 providing connections for the signal lead wires 6-10 coming from the precordial electrodes V2, V3, V4, V5 and V6. The rectangular connector elements 142 in the lower row are for the lower parts of connector elements 11 providing connections for the lead wire shields SH. Connector elements 142 provide a shielding ground connection 39, which in this case is the amplifier ground G. The dual connector elements 11 are consistent with the AAMI standard.
The function of the N-electrode is to equalize the potentials of the patient and the amplifier. For this purpose, the N-electrode is traditionally connected to the amplifier ground G either directly or through a single resistor or a network of resistors and capacitors. The potential difference may be further reduced with an electronic circuit 34 actively driving the potential of the patient closer to the potential of the amplifier, often referred to as right-leg-drive circuit (RLD). This kind of driver includes a current limiting circuit typically adjusted to a maximum current allowed in a single-fault condition (EN60601-1).
Resistors R protect the preamplifiers 35, RLD-circuit 34, and impedance measurement circuit 33 from over-voltages and limit the currents through electrodes in case of defibrillation.
Also in prior art, patent application WO 01/06923 discloses a lead set and connector arrangement system as shown in FIG. 3, which can operate both as a 5-lead measuring system and as a 12-lead measuring system, and in which it is possible to combine the limb-electrode and precordial electrode parts of the collecting connector 13, allowing the same amplifier unit to function alternatively in a 5-lead measuring system with shielded lead wires 37 or in a 12-lead measuring system with unshielded lead wires 38. In this system a small and lightweight collecting connector can be used.
The circuitry for measuring 5 or 12-lead ECG is presented in FIG. 4. In this arrangement, when measuring 12-lead ECG, the circular second connector elements 141 are used for the first connector elements 11 of the lead wires 1-5 coming from the limb electrodes R, F, L, N, and one precordial electrode V1 (un-shielded 12-lead set 38). Connected to the rectangular second connector elements 142 of the lower row are the first connector elements 11 of the lead wires 6-10 coming from the precordial electrodes V2, V3, V4, V5 and V6 (unshielded 12-lead set 38).
In case of 5-lead ECG one possible lead-set configuration consists of AAMI-compatible connectors with shields connected to the lower, rectangular connectors 142 (shielded 5-lead set 37). In this case, analog switches 18-22 inside the amplifier unit may be used to provide shielding ground connection 39 for the lower connector pins 142 and hence for the lead wire shields SH.
Use of shielded signal lead wires has an importance especially in ECG monitoring performed during anesthesia, because surgical operations are often performed using a so-called diathermy device, i.e. an electric surgical knife, whose high-frequency electric current would otherwise confuse the ECG monitoring process, but also in intensive care monitoring, where line voltage interference and electrostatic voltages coupling directly to the lead wires may be a significant problem.
Considering the prior art, there remain unresolved technical problems related to the use of the analog switches 18-22 for providing shielding ground connection for the lead wire shields. When changing over from 5 to 12-lead measuring mode by adding precordial electrodes, a low-impedance current path is created between the right-leg-drive circuit 34 and amplifier ground G, if the precordial electrodes get connected to the amplifier ground G by via analog switches 18-22. Because of small offset voltages always present in this kind of electronics circuit, the output of the RLD circuit may be driven to the current limit, which makes the RLD circuit effectively inoperative. Furthermore, because connection of electrodes and lead wires in this manner is a normal operating procedure, the current limit of the RLD circuit has to be set to conform with the 10 μA maximum allowed current during normal operation (EN60601-1), thus limiting the efficiency of the circuit compared to the typically used 50 μA limit.
Considering the prior art, there also remains an unresolved technical problem relating to the changeover from 5- to 12-lead measuring mode in case of the combined collecting connector. When the ECG electrodes are initially attached to the patient and the analog switches 18-22 are open, the ECG device is capable of deducing the number of electrodes used by analyzing the signal characteristics from the different signal lines. Once the analog switches are closed, it is not possible to detect addition of precordial electrodes based on signal characteristics, there thus being no way for the amplifier to detect the addition of the precordial electrodes and this way to automatically change over from 5 to 12-lead measuring mode.
To mitigate the above-mentioned shortcoming, patent application WO 01/06923 discloses a method of detecting the addition of precordial electrodes based on a special arrangement related to the shield of the N electrode lead wire, which corresponds to the V6 signal lead wire. This arrangement is not optimal because in this case the moment of change from 5 to 12-lead measuring mode in the monitor depends on the order in which the precordial electrodes are connected to the patient.
One possible solution would be to use the activation of the RLD circuit current limiting feature as an indicator of addition of a precordial electrode. This solution is not desirable, since it would be based on the relatively low 10 μA current limit setting. Furthermore, the connection of a precordial electrode would make the RLD circuit temporarily inoperable.